The invention relates to a distance-measuring method, a distance measuring apparatus and a computer program product.
In the area of electronic distance measurement, various principles and methods are known, the two fundamental optoelectronic principles of measurement being embodied by phase meter and transit time meter. Both principles have advantages and disadvantages and are used in a multiplicity of geodetic devices. Thus, for example, the measuring means used in geodetic surveying, such as theodolites or tacheometers, are equipped mainly with phase meters since these have the advantage of high accuracy and compact design, which facilitates integration in a telescope.
In the phase measurement technique, light pulses (usually square-wave signals) are emitted with repetition frequencies in the range from a few MHz to a few hundred MHz. In addition to LEDs, conventional CW laser diodes having peak powers of a few mW can also be used as light sources for this purpose. The mean emitted energy is sufficiently high, and the visibility of the laser spot on the target is not a problem for the distances to be measured using phase meters.
For the distance measurement, the phase position of the emitted signal is compared with that of the returning signal. The phase shift is proportional to the measured distance. The RF signal received by a photodiode is amplified and is down-converted true to phase to a lower frequency band with the aid of a phase locked loop (PLL)-controlled local oscillator signal.
Instead of high-frequency signal sampling with sampling rates in the GHz range, it is substantially easier to employ a low-frequency receiver signal. Here, the sampling and analogue-digital conversion in the low-frequency (LF) range is simpler, more accurate and less current-consuming by orders of magnitude. In conventional phase meters, only the basic harmonic or the down-converted LF signal is used.
In order to extend the un-ambiguity of a phase-measuring system from the meter range into the km range, one or more coarse distance measurements with lower modulation frequencies are usually carried out in addition to the fine distance measurement.
For achieving sufficient absolute accuracy, an internal light path (calibration or reference path) and an external light path (measuring path) are usually measured in succession. In this way, changes in transit times can be calibrated in the electronics. The calibration of the transit time changes can also be realized by means of two identical, parallel receiving channels. Accurate distance measurement is possible in the case of phase meters only with 2 channels with high signal separation.
The advantages of the phase meter are in particular the simple design, the measurement at LF level and the reliable beam resources available.
The falsification of the measured distance by the superposition of signals due to the optical crosstalk proves to be disadvantageous, so that pronounced channel separation of high suppression is required. An accurate distance measurement therefore requires rigorous signal separation between transmitting channel and receiving channel, which is very difficult to achieve, complex and expensive in a telescope of compact design. In addition, only one target should be in the measuring beam, since otherwise errors in the coarse distance measurement can also occur in addition to fine distance measuring errors. For longer distances, both at least one coarse measurement and one fine measurement are required. Single-channel measuring principles, i.e. those without light path or channel switch, are not possible with the simple frequency concept.
Transit time meters do not have the disadvantage of rigorous signal separation but their accuracy of measurement is often insufficient for geodetic surveying, in particular if sub-mm accuracies are required.
In the case of rangefinders which operate according to this principle, a light pulse is likewise emitted, this light pulse being divided by suitable optical measures so that a part is passed via an internal light path (calibration path) directly to the receiver whereas the remaining component of the light is sent from the device via the external light path.
This external component strikes a target some distance away—the distance to be measured (=measured distance)—and is reflected back from there and passed via a suitable optical system to the same receiver, the receiver expediently being a photodiode with down-circuit amplifiers.
The light pulse passed via the internal light path produces in the receiver a reference pulse, which is referred to below as start pulse. The light pulse passed via the external light path (measured distance) produces in the receiver the so-called measured pulse, which is referred to below as stop pulse.
Since the length of the internal and the external light path are different, the two light pulses arrive at the receiver at different times. The time difference between start pulse and stop pulse is referred to as transit time and is proportional to the difference in length between internal and external light paths. The time differences to be measured are very small, i.e. they must be determined extremely accurately in order to arrive at a geodetic accuracy of mm or sub-mm suitable for a usable distance measuring system. For determining transit time, at least the received signal is digitized, for which purpose very complicated high-frequency electronic circuits with sampling rates in the GHz range are required.
Further light pulses are emitted by the transmitter only after the stop pulse has arrived at the receiver. This requires a relatively low pulse repetition frequency of a few 10 kHz in order to be able to ensure an unambiguity of a few km. In order to be able to emit sufficiently great light energy at such a low pulse repetition frequency so that the laser spot is readily visible or so that it is possible to go to the limit of eye safety (laser class 2), the peak power must be in the range from several 10 W to 1 kW depending on pulse width.
Advantages of the single-channel transit time measurement are the absence of time drift because start and stop pulse take place shortly in succession and are subject to the same transit times, the insensitivity to optical crosstalk because the stop pulse takes place only after the crosstalk pulse, and the omission of the unnecessary switchable optical components for the internal and external light path.
However, the disadvantages of the transit time measurement are in particular the very complicated sampling and the time measurement of the RF signals and the complicated beam sources, which are also difficult to handle (e.g. microchip lasers with quality modulation). Semiconductor laser diodes having high peak power have a disadvantageously extensive illumination area, and the radiation can be focused or collimated only to an insufficient extent. The laser beam can be focused to a quasi-parallel pencil with sufficiently small divergence only with sufficiently spatially coherent point light sources which emit from a diffraction-limited, small area. The semiconductor laser diodes which emit from such a diffraction-limited small area and can therefore be focused to a beam with sufficiently small divergence have to date a peak transmission power which is limited to a few hundred mW and is therefore much too low for a pulse transit time meter.
Although various arrangements which manage without channel separation and light switching are known, all solutions are associated with various disadvantages.
A method and an apparatus for optoelectronic distance measurement according to the phase measurement principle are described in the document DE 100 06 493 C2. The phase meter is equipped with a 2-channel receiver without mechanical light path switching, the circuit being equipped with 2 photoreceivers. In a distance measurement, in each case the signal phases are measured at the first and at the second receiver. The measured phase at the first receiver describes the distance of the internal reference light path, and the phase at the second receiver describes the distance to the target object. The difference between the two phases gives the drift-free absolute distance based on the reference light path. With a second transmitter, any phase difference produced via the 2 photo receivers and the amplification circuits thereof can be simultaneously measured. Disadvantages of this solution are both the two transmitting units and the two photoreceivers, which result in a more complex construction, and the interleaving of the two light paths by means of two elements for beam combination for each of the two photoreceivers.
A second arrangement is described in the document U.S. Pat. No. 6,369,880. The phase meter disclosed there is equipped with a 2-channel receiver without mechanical light path switching and with two photoreceivers. In a distance measurement, in each case the signal phases at the first and at the second receiver are measured, the difference between the two phases corresponding to the measured distance. A disadvantage of this solution is likewise the duplication of the photosensitive and phase-sensitive receiving unit.
WO 03/069779 describes a transit time meter having a 2-channel receiver without mechanical light path switching so that the reference measurement principle free of optical switching was realized in the case of transit time meters too. However, the transit time meter disclosed likewise uses 2 photoreceivers. The signals of the 2 receivers are fed to a time-measuring unit operating in the high frequency range. In a distance measurement, the difference between the internal and external transit times measured in parallel is calculated. This solution, too has the disadvantage of the duplication of the receiving unit.
Thus, the solutions of the prior art require a switching mechanism between external and internal light path or a duplication of the receiving system and are therefore expensive and complex in terms of design.
DE 10112833 C1 describes a method and an apparatus for electrooptical distance measurement which is intended to combine the advantages of a phase transit time method with those of a pulse transit time method, high peak light powers, i.e. a good signal/noise ratio, being of primary interest in the case of the latter. For the electrooptical distance measurement, the laser beam of an emitter diode is sent as an intensity-modulated sequence of transmitted light pulses to a target plate-free measured object, and the measuring light pulses reflected there are detected by a light detector, by which a first photocurrent component is generated. In addition, a small fraction of the intensity-modulated transmitted light pulse sequence is branched off as a reference light pulse sequence and, after passing over a known reference path, is likewise passed to the light detector, with the result that a second photocurrent component is produced. The light detector used is an avalanche photodiode in which the superposed measured light pulses are directly converted with a mixer pulse sequence produced by a local oscillator into a comparatively low intermediate frequency range, from which the measured distance can be determined after appropriate conversion.
A difficulty of this approach is that start pulse and stop pulse may overlap so that, in this case, separation or assignment of the pulses is not possible. Because the number of harmonics used is 20, frequencies into the gigahertz range are necessary. A reduction of the harmonics used would lead to broad pulses, which in turn increases the probability of pulse overlap.
An object of the invention is to provide a method and a device for distance determination with reduced complexity and technical effort, respectively.
A further object of the present invention is to combine advantages of phase and transit time principles without having to accept disadvantages thereof, and in particular to permit the separability of pulses.